Integrated circuits are often provided with debug circuitry which allows the integrated circuit to be debugged. The integrated circuit usually comprises a bus and a plurality of modules connected to the bus which put packets onto the bus. The debug circuitry is one of these modules. The modules also usually include a CPU. In order to operate, the debug circuitry is arranged to receive information from an external tool, put that information onto the bus and to check the response to that information or to output the response to the external tool. The debug circuitry can also carry out internal checks within the integrated circuit.
However, these known circuits have a problem. If the external circuitry identifies that there is a problem with a module, it is difficult to identify what has caused the problem in that module. This is because the module issuing the information causing the difficulty will continue to put information onto the bus. This means that the information in the module may have significantly changed by the time that the module is looked at. This makes it difficult to ascertain why the module in question has caused the problem.